Choice of a catalyst and technological scheme for synthesis of solketal

Article abstract of DOI:10.1134/S1070427216100094

Results are presented obtained in tests of various homogeneous and heterogeneous catalysts in a synthesis of solketal (2,2-dimethyl-4-hydroxymethyl-1,3-dioxolane). It is shown that, all other conditions being the same, the highest catalytic activity is observed for sulfuric acid and beta zeolite. Data are presented on how the equilibrium constant of the reaction of glycerol ketalization depends on temperature. A technological scheme of the process for synthesis of solketal is suggested and its description is presented.

Full text of DOI:10.1134/S1070427216100094

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2016, Vol. 89, No. 10, pp. 1619−1624. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © G.S. Dmitriev, A.V. Terekhov, L.N. Zanaveskin, S.N. Khadzhiev, K.L. Zanaveskin, A.L. Maksimov, 2016, published in Zhurnal Prikladnoi
Khimii, 2016, Vol. 89, No. 10, pp. 1298−1304.
CATALYSIS
Choice of a Catalyst and Technological Scheme
for Synthesis of Solketal
G. S. Dmitriev*, A. V. Terekhov, L. N. Zanaveskin, S. N. Khadzhiev,
K. L. Zanaveskin, and A. L. Maksimov
Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences,
Leninskii pr. 29, Moscow, 119991 Russia
*e-mail: dmitriev.gs@mail.ru
Received October 27, 2016
Abstract—Results are presented obtained in tests of various homogeneous and heterogeneous catalysts in a
synthesis of solketal (2,2-dimethyl-4-hydroxymethyl-1,3-dioxolane). It is shown that, all other conditions being
the same, the highest catalytic activity is observed for sulfuric acid and beta zeolite. Data are presented on how
the equilibrium constant of the reaction of glycerol ketalization depends on temperature. A technological scheme
of the process for synthesis of solketal is suggested and its description is presented.
DOI: 10.1134/S1070427216100094
The interaction of glycerol wit acetone on acid-type
catalysts yields two products: 2,2-dimethyl-4-hydroxy-
methyl-1,3-dioxolane, also named solketal, and 2,2-di-
methyl-1,3-dioxan-5-ol. The amount of the latter in the
reaction products commonly does not exceed 2–3%. The
scheme of the occurring reactions is presented below:
In what follows, the term solketal will mean a mixture
of these two isomers.
Solketal belongs to oxygenates, a group of lower
alcohols and esters used as high-octane components
of motor fuels. Use of oxygenates extends the service
characteristics of fuels and, as a rule, improves their
2,2-Dimethyl-4-hydroxymethyl-1,3-dioxolane
(1)
Glycerol (GL)
Acetone (AC)
2,2-Dimethyl-1,3-dioxan-5-ol
1619

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DMITRIEV et al.
The reactor was charged with the following amounts
X, %
of the reagents (g): glycerol 25.0, ethanol 19.0, and
acetone 80.0. The acetone : glycerol molar ratio was 5 : 1.
When being agitated, the reaction mixture was heated to
a temperature of 40°C. The instant of time at which the
catalyst was introduced into the reaction mixture was
considered the reaction onset time. The reaction mixture
was periodically sampled to analyze the reaction products.
The samples of the reaction mixture, taken for
analysis, were neutralized with a 0.1 N NaOH solution
at the instant of sampling. The reaction products were
analyzed by gas-liquid chromatography on a Tsvet-800
chromatograph with a ZB-WAX capillary column and
flame-ionization detector.
τ, min
Fig. 1. Conversion X of glycerol in the ketalization reaction on
various catalysts vs. time τ. (1) Sulfuric acid, (2) beta zeolite
AOAZKiOS, (3) beta zeolite C814E, (4)Amberlist 35 dry ion-
exchange resin, (5) KU-2-8 ion-exchange resin.
RESULTS AND DISCUSSION
All experiments on syntheses of solketal were per-
formed by taking the amount of catalysts that provided the
same and constant concentration of acid centers, equal to
2.3 mmol of H+ per 124 g of the reaction mixture. The data
on the concentration of acid centers in the heterogeneous
catalysts were taken from their rating data. The results of
these experiments are presented in the table.
quality. Gasolines with oxygenates have improved de-
tergent properties, combustion characteristics, and form
a smaller amount of carbon monoxide and hydrocarbons
in burning. In recent years, the increasing popularity has
been gained by oxygenates produced from alternative
raw materials and, in particular, from glycerol, a waste
formed in production of a biodiesel fuel [1–3]. Solketal
belongs to just this kind of products.
The experiments demonstrated that, at the same num-
ber of acid centers in the reaction mixture, the highest
rate of glycerol ketalization is provided by sulfuric acid.
Reaching this result required the smallest amount of the
catalyst: the content of sulfuric acid in the reaction mass
was 0.095 wt %.
The present communication is devoted to choosing
the most active and technologically convenient catalyst
for synthesis of solketal and developing a technological
scheme for obtaining this product.
Beta zeolite (AO APCOS, Angarsk) was found to
be the most active heterogeneous catalyst. At the same
concentration of acid centers and amount of the catalyst
in the reaction mass as those for beta zeolite CP814E
(Zeolyst International), the initial rate of the reaction in
which solketal is formed on Angarsk zeolite was nearly
3.5 times that on the beta zeolite CP814E. Amberlist 35
dry ion-exchange resin (Dow company) also demonstrated
comparatively high initial rate and throughput, being only
slightly inferior to Angarsk zeolite.
EXPERIMENTAL
We used the following reagents: glycerol (special
purity), acetone (chemically pure), rectified ethanol,
and distilled water. As catalysts served sulfuric acid
(chemically pure), Amberlist 35 dry (Dow), KU-2-8
cation-exchange resin preliminarily dried with acetone
on a water bath, and beta zeolite CP814E (Zeolyst
International).
The study was carried out in a glass reactor with
reflux, cartridge for a thermocouple, and ground joint
for introduction of a catalyst and sampling. The reaction
mixture was agitated with a magnetic rabble. The
temperature in the reactor was adjusted and measured
with a set of devices including a TRM-101 single-channel
microprocessor programmable temperature controller-
meter and a solid-state relay, connected to the heating
element of the magnetic rabble.
The data obtained suggest that the activity of the hetero-
geneous catalysts in the synthesis of solketal is determined
by the nature of acid centers on the catalyst surface, by the
fabrication conditions, and, presumably, by the composi-
tion and amount of impurities in the catalysts, rather than
by the specific concentration of the acid centers.
Figure 1 shows how the conversion of glycerol
into solketal depends on the experiment duration. The
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CHOICE OF A CATALYST AND TECHNOLOGICAL SCHEME
Initial rates of solketal synthesis on various catalysts
1621
Concentration of acid
Charged catalyst
mass, g
Initial output
Catalyst
Initial rate, M s–1
centers, mmol H+/gcat
capacity, M s–1 gcat
–1
H2SO4
19.6
1.1
0.118
2.10
3.48 × 10–2
7.61 × 10–3
0.295
Beta zeolite, AO APCOS,
Angarsk
3.6 × 10–3
Beta zeolite CP814E, Zeolyst
International
1.1
2.10
2.19 × 10–3
1.0 × 10–3
KU-2-8
4.6
5.0
0.50
0.46
3.00 × 10–4
1.48 × 10–3
0.6 × 10–3
3.2 × 10–3
Amberlist 35 dry
amounts of catalysts used in synthesis are listed in the
table.
with a fixed catalyst bed. The mixture of the starting
reagents is delivered into the upper part of the reactor,
with the catalyst bed uniformly sprayed. The reaction of
glycerol with acetone occurs in a film flowing down over
the surface of catalyst particles. However, the starting
emulsion stratifies in the upper part of the reactor, in
which the conversion of glycerol is still lower than 40%,
and a glycerol film hindering the approach of acetone to
the active surface of the catalyst is formed on its surface
[4].
It follows from Fig. 1 that sulfuric acid and Angarsk
beta zeolite make it possible to reach, already during
the first 15–20 min, a 75–76% conversion of glycerol,
which is the maximum and equilibrium conversion of
glycerol in the reaction with acetone. This conversion is
only observed in 50 min on beta zeolite CP814E and in
nearly 110 min on Amberlist 35 dry ion-exchange resin.
Thus, the following two catalysts can be recommended
for industrial manufacture on the basis of the experimental
data: beta zeolite produced by AO Angara Plant of
Catalysts and Organic Synthesis and sulfuric acid.
For an apparatus with fixed catalyst bed to operate with
high efficiency, it is necessary to provide the maximum
possible wetting of its surface, which is commonly
reached at spraying densities of 3–5 m3 m–2 h–1. However,
experiments demonstrated that, even at a spraying density
of 0.15 m3 m–2 h–1, the glycerol film on the catalyst
surface becomes so thick and flows down so slowly that
this leads to flooding of the catalyst and to accumulation
of an emulsion of starting reagents over its bed. Thus,
it is nearly impossible to perform the process without a
preliminary homogenization of the mixture of the starting
reagents.
It is known that heterogeneous catalysts have,
compared with homogeneous catalysts, a number of
advantages, including the low expenditure of a catalyst
per unit amount of the target product, absence of
wastewater, no need to purify the reaction mass to remove
the catalyst: zeolite is comparatively easily separated from
the reaction mass and does not corrode the apparatus. This
circumstance justified the development of a technological
scheme for synthesis of solketal on the heterogeneous
catalyst, beta zeolite.
This problem is, as a rule, solved by introduction
into a binary mixture of a third component raising the
mutual solubility of glycerol and acetone. In synthesis
of solketal, the role of this additive is played by ethanol,
which homogenizes the mixture of the starting reagents,
yields no by-products, and can be easily evaporated from
the reaction mixture and be recycled into the synthesis
stage. It was found experimentally that the glycerol :
ethanol ratio should not be less than 1 : 1.
A specific feature of the synthesis of solketal is the
low mutual solubility of the starting reagents, acetone and
glycerol. As a result, the starting mixture of the reagents
is heterophase at a glycerol : acetone molar ratio of 5 :
1 and rapidly stratifies without a vigorous agitation.
The reaction mass becomes homogeneous only when
containing increased contents of solketal and water ,
which corresponds at a temperature of 40°C to a glycerol
conversion X ≥ 40%.
However, using ethanol as a homogenizing reagent
will complicate the technological scheme of the process
due to the installment of a mixing apparatus at the inlet
The most efficient apparatus for a heterogeneous-
catalytic synthesis of solketal is a column reactor packed
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1622
DMITRIEV et al.
probability, even distilled glycerol contains trace amounts
of mineral salts, fatty acids, and glycerol esters [8, 9].
In addition, side reactions of the aldol condensation of
acetone may occur on acid centers of the catalysts, which
also poisons the catalyst [10].
Thus, the recycled glycerol and products of side
reactions comparatively rapidly deactivate a catalyst
and causes its replacement. Because the cost of beta
zeolite is several thousand roubles per kilogram, the need
to regularly replace the catalyst adversely affects the
economic attractiveness of this technology for synthesis
of solketal.
1/T, K–1
Fig. 2. Dependence of the equilibrium constant Keq of the
reaction of solketal synthesis on temperature T.
In contrast to that described above, the technology for
solketal synthesis with the use of a homogeneous catalysis
does not require an additional component for homog-
enization of the system. The starting reagents (glycerol,
acetone, and sulfuric acid) are continuously delivered into
the mixing reactor. The residence of the reaction mixture
in the mixing apparatus for more than 10–15 min (at a
sulfuric acid concentration of 0.1 wt %) is sufficient for
reaching the 40–50% conversion, which guarantees that
a homogeneous mixture of reaction products is formed.
Because the reaction mixture is homogeneous and
contains a catalyst, the reaction can further occur even
without agitation until the equilibrium is attained.
of the reactor for synthesis of solketal. In addition, the
introduction of a third component into the system will
cause a lower rate of the solketal formation reaction
due to the decrease in the concentration of the starting
reagents and will require an additional energy expenditure
in the stage in which ethanol is separated from the target
product.
It is known that the reaction of glycerol ketalization is
of equilibrium type, which results in that the conversion
of glycerol in industrial technologies does not exceed
75–80%. To improve the economic efficiency and solve
ecological problems, the technological scheme should
include the stage in which the unreacted glycerol is
isolated to be recycled into the synthesis stage. Because
glycerol is the highest boiling product, it will be collected
in the still product of the third rectification column in the
stage of separation of the reaction mass. Consequently,
this glycerol will contain all tarry products formed in the
stills of al the three columns and also the technological
“garbage,” including the products of equipment corrosion,
parts of liners, and machinery oil that can penetrate into
the system from stirrers, pumps, etc.
One of possible variants is to accumulate and store
reaction products in vessels. This is quite a reasonable
solution because industrial plants nearly always have
a gap between vessels used in different stages of the
process.
It should be noted that the equilibrium constant of
the reaction in which glycerol interacts with acetone
grows with decreasing temperature. Figure 2 shows
the experimentally obtained temperature dependence
of the equilibrium constant of the reaction of glycerol
ketalization. The dependence is described by the equation
The recycling of this glycerol into the synthesis
reactor packed with a heterogeneous catalyst will lead to
a gradual deactivation of the catalyst due to the adsorption
of above impurities on its surface.
1
ln Keq = 1745.8 –– – 6.0.
(2)
T
Using the Van’t Hoff’s isobar equation, we calculated
from the data in Fig. 2 the heat effect of the reaction to
be ΔH = –14.5 kJ mol–1.
Experiments demonstrate that, even in operation
without glycerol recycling, the activity of heterogeneous
catalysts decreases in the course of time [5–7]. The rate of
this process directly depends on the quality of glycerol.
If raw glycerol containing mineral salts, soaps, and fatty
acids is used, the deactivation of the catalyst is very fast
[5]. However, these processes are inevitable even in the
case of a distilled glycerol [6, 7]. The reason is that, in all
The reaction is exothermic. When solketal is syn-
thesized in a 5-fold molar excess of acetone relative to
glycerol, the released heat is sufficient for heating the
reaction mixture by at most 12° (with the loss of heat
disregarded).
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CHOICE OF A CATALYST AND TECHNOLOGICAL SCHEME
1623
circulated acetone evaporated in the first rectification
column should not exceed 1.3 wt %. With consideration
for the fact that the process is performed at a 5-fold molar
excess of acetone and a large reflux ratio of 4 is required in
its rectification, more than 80% of the energy expenditure
by the system for separation of the reaction products is
accounted for by the column for evaporation of acetone.
Prior to being separated into reaction products, the
reaction mixture containing a catalyst, sulfuric acid, is
neutralized.
T, °C
Figure 4 shows the flow diagram of the process in
which solketal is synthesized.
Fig. 3. Dependence of the equilibrium conversion Xeq of
glycerol on temperature T. Glycerol : acetone molar ratio =
1 : 5, starting glycerol of D-98 brand.
Previously, the results of a study of an alternative
way to recover solketal from reaction products by the
extraction method have been reported [12]. It was shown
that, if acetone is evaporated from reaction products,
solketal can be extracted from the remaining mixture
of components with benzine containing 30–35 wt %
aromatic compounds. In this case, the content of solketal
in the resulting benzine will be no less than 4.5 wt %.
Despite that this method of obtaining benzine enriched
with solketal is attractive, there are several reasons why
its implementation is impractical. First, the whole amount
of acetone is to be evaporated prior to the extraction of
solketal. Because the rectification of acetone consumes
most part of energy, the recovery of solketal from
Because the equilibrium conversion of glycerol grows
as temperature is lowered (Fig. 3), it is reasonable to cool
the vessels used to keep the reaction products.
The reaction products are separated in a set of rectifi-
cation columns. The excess acetone is evaporated in the
first rectification column and recycled into the synthesis
stage. Water is evaporated in the second rectification col-
umn, and the commercial product, solketal, in the third
column in a vacuum.
Because the reaction in which solketal is synthesized
is of the equilibrium type, the content of water in the
Acetone
Acetone
(recycled)
Glycerol
R1
Water
Solketal
C1
C2
C3
Glycerol
V1
To utilization
Glycerol
(recycled)
Fig. 4. Flow diagram for manufacture of solketal. R1 designates the reactor; V1, vessel; C1–C3, rectification columns.
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DMITRIEV et al.
The synthesis of solketal in the presence of sulfuric
acid is the most reasonable and economically efficient
technology for production of this compound. A flow
diagram for synthesis of solketal was suggested and its
description was given.
reaction products by the extraction method and that by
the rectification on three columns are approximately the
same in energy expenditure.
Second, the recovery of solketal by the extraction
method requires large flows of benzine in the technological
scheme and, accordingly, a rather large number of vessels
for the starting benzine and that enriched with solketal.
And this entails not only a gross expenditure of energy and
materials, but also a high fire hazard of the installation. In
view of these data, the technological scheme for recovery
of solketal by the extraction method is not considered an
industrially usable technology.
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The experiments performed in the study demonstrated
that two catalysts can be used for industrial production of
solketal: beta zeolite manufactured y AO Angarsk Plant
of Catalysts and Organic Synthesis and sulfuric acid.
The technological scheme based on the application of a
heterogeneous catalyst has a number of disadvantages,
the main of which are the following: need to introduce
and additional component (ethanol) for homogenizing the
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